There are numerous situations wherein it is desirable to know if there is an undesired or unauthorized impedance to ground on internal and/or external wiring of electrical power supply systems and to provide an alarm signal when ground faults are detected. Ground fault detection systems are used to protect equipment and/or personnel in a wide variety of applications and to assure continuous and accurate operation of the equipment. One typical example of a power supply system which it is vital to maintain in operative condition is the pair which provides power to alarms which sound in response to the detection of fire, smoke or other abnormal conditions.
Because the detection of ground faults is important, a wide variety of techniques have been developed for their detection. Some prior art techniques, particularly those employing relays, were not sufficiently sensitive. Prior art solid state techniques often tended to be too sensitive and responded to noise or line fluctuations. In some applications it is desirable to have symmetrical sensitivity to both sides of the power supply. Many prior art systems failed to provide the desired symmetrical sensitivity. Some systems were more sensitive than desired with respect to capacitance between system wiring and ground. Also, some prior art systems had an inherent threshold of sensitivity and could not easily be adjusted to be more or less sensitive to meet the requirements of a particular application.